Open Paleo

Hylaeosaurus

Mantell, 1833 hy-LEE-o-SOR-uhs /haɪˌliːoʊˈsɔːrəs/

From Greek hylaios (of the forest, woodland) and sauros (lizard); meaning 'woodland lizard' or 'forest lizard', named because its remains were found in the Tilgate Forest area of England

137.1–132.6 million years ago Valanginian, Early Cretaceous
Early Cretaceous stages

Hylaeosaurus is a herbivorous ankylosaurian dinosaur that lived about 136 million years ago, in the late Valanginian stage of the early Cretaceous period of England. It was found in the Grinstead Clay.

Diagnosis

  1. Sharp ~120° angle between the acromial process and the proximal plate of the scapula, to a greater extent than in Hoplitosaurus.
  2. Acromial process of the scapula shelf-like (not thumb-like or folded over), projecting ventrolaterally from the dorsal third of the scapular blade.
  3. Dorsal edge of the proximal plate of the scapula curving laterally.
  4. Anteroposteriorly trending ridge at mid-height on the lateral surfaces of the cervical centra.
  5. Centrodiapophyseal fossa of the dorsal vertebrae strongly concave, with strong anterior and posterior centrodiapophyseal laminae separating it from the prezygapophyseal and postzygapophyseal centrodiapophyseal fossae.

Species

Hylaeosaurus armatus Mantell, 1833

Type species

From Latin 'armatus' meaning 'armed' or 'armored', in reference to the spikes and armor plates of the holotype.

Holotype
NHMUK PV OR 3775 , Natural History Museum
Material
Partial articulated skeleton: poorly preserved skull elements (maxilla, jugal, postorbital, dentary, ?occipital condyle), seven cervical and four dorsal vertebrae, cervical and dorsal ribs, both scapulae and coracoids, and numerous osteoderms.
Notes
Raven and colleagues (2020) give the holotype as NHMUK PV OR3775; earlier papers write it R3775.
Horizon
Grinstead Clay Formation, Valanginian, Early Cretaceous , 137.1–132.6 million years ago
Locality
Tilgate Forest, near Cuckfield, West Sussex, United Kingdom Mantell (1833) names the strata of Tilgate Forest and no formation; Raven and colleagues (2020) supply the Grinstead Clay and describe the site as imprecisely located.

Synonyms of the genus

Name Type Reason
Hylosaurus junior objective Variant spelling treated as a junior synonym of Hylaeosaurus.
Vectensia nomen nudum Informal name; never met ICZN requirements for formal publication and is treated as referable to Hylaeosaurus. Per Olshevsky's Dinosaur Genera List.

References

  1. Mantell, G. A. (1833). The geology of the south-east of England. Longman, Rees, Orme, Brown, Green, & Longman. doi:10.5962/bhl.title.106921 mantell1833a
  2. Mantell, G. A. (1833). Observations on the remains of the Iguanodon, and other fossil reptiles, of the strata of Tilgate Forest in Sussex. Proceedings of the Geological Society of London 1: 410-411.

    Abstract of Mantell's account of the Tilgate Forest skeleton, for which he suggests the name Hylaeosaurus.

    mantell1833b
  3. Raven, T. J.; Barrett, P. M.; Pond, S. B.; Maidment, S. C. R. (2020). Osteology and taxonomy of British Wealden Supergroup (Berriasian-Aptian) ankylosaurs (Ornithischia, Ankylosauria). Journal of Vertebrate Paleontology. doi:10.1080/02724634.2020.1826956

    Redescribes the Hylaeosaurus holotype NHMUK PV R 3775, restricts the hypodigm to the holotype, revises the diagnosis, and confirms it as a valid Valanginian taxon.

    raven2020a
  4. Jones, B. (2026). The Princeton Encyclopedia of Dinosaurs: Ornithischians. Princeton University Press.

    Source of the hip height estimate, and the appearance features.

    jones2026a